1 ©Mohamed Khedr., 2008
EC 551Telecommunication System Engineering
Mohamed Khedrhttp://webmail.aast.edu/~khedr
2 ©Mohamed Khedr., 2008
Syllabus� Tentatively
WiMax MAC LayerWeek 6
WLAN Physical LayerWeek 7
WLAN MAC LayerWeek 8
Cellular Communication ConceptWeek 9
FDMA, TDMA, CDMA and DuplexingWeek 10
GSM SystemWeek 11
GPRS SystemWeek 12
UMTSWeek 13
VOIPWeek 15
IP networksWeek 14
WiMax Physical LayerWeek 5
Coding techniques in wireless systemsWeek 4
OFDM and modulation techniquesWeek 3
Wireless Channel characteristicsWeek 2
OverviewWeek 1
3 ©Mohamed Khedr., 2008
Multipath: Time-Dispersion => Frequency Selectivity
� The impulse response of the channel is correlated in the time-domain (sum of “echoes”)� Manifests as a power-delay profile.
� Equivalent to “selectivity” or “deep fades” in the frequency domain� Delay spread: τ ~ 50ns (indoor) – 1µs (outdoor/cellular).� Coherence Bandwidth: Bc = 500kHz (outdoor/cellular) – 20MHz (indoor)� Implications: High data rate: symbol smears onto the adjacent ones (ISI).
Multipatheffects
~ O(1µs)
4 ©Mohamed Khedr., 2008
Doppler: Dispersion (Frequency) => Time-Selectivity� The doppler power spectrum shows dispersion/flatness ~ doppler spread (100-200 Hz for
vehicular speeds) � Equivalent to “selectivity” or “deep fades” in the time domain correlation envelope. � Each envelope point in time-domain is drawn from Rayleigh distribution. But because of
Doppler, it is not IID, but correlated for a time period ~ Tc (correlation time). � Doppler Spread: Ds ~ 100 Hz (vehicular speeds @ 1GHz) � Coherence Time: Tc = 2.5-5ms.� Implications: A deep fade on a tone can persist for 2.5-5 ms! Closed-loop estimation is valid
only for 2.5-5 ms.
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Fading Summary: Time-Varying Channel Impulse Response
� #1: At each tap, channel gain |h| is a Rayleigh distributed r.v.. The random process is not IID.
� #2: Response spreads out in the time-domain (τ), leading to inter-symbol interference and deep fades in the frequency domain: “frequency-selectivity” caused by multi-path fading
� #3: Response completely vanish (deep fade) for certain values of t: “Time-selectivity” caused by doppler effects (frequency-domain dispersion/spreading)
6 ©Mohamed Khedr., 2008
Fading: Jargon� Flat fading: no multipath ISI effects.
� Eg: narrowband, indoors� Frequency-selective fading: multipath ISI effects.
� Eg: broadband, outdoor.
� Slow fading: no doppler effects. � Eg: indoor Wifi home networking
� Fast Fading: doppler effects, time-selective channel� Eg: cellular, vehicular
� Broadband cellular + vehicular => Fast + frequency-selective
7 ©Mohamed Khedr., 2008
Power Delay Profile => Inter-Symbol interference
� Higher bandwidth => higher symbol rate, and smaller time per-symbol� Lower symbol rate, more time, energy per-symbol� If the delay spread is longer than the symbol-duration, symbols will “smear” onto adjacent
symbols and cause symbol errors
Symbol Time
Symbol Time
path-2path-3
path-1
Path Delay
Pow
er
Delay spread~ 1 µµµµs
Symbol Error!If symbol rate ~ Mbps
No Symbol Error! (~kbps)(energy is collected
over the full symbol periodfor detection)
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Multipath Fading Example
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Key Wireless Channel Parameters
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Fading: Design Impacts (Eg: Wimax)
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Orthogonal Frequency Division Multiplexing
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Motivation
• High bit-rate wireless applications in a multipath radio environment.
• OFDM can enable such applications without a high complexity receiver.
• OFDM is part of WLAN, DVB, and BWA standardsand is a strong candidate for some of the 4G wireless technologies.
13 ©Mohamed Khedr., 2008
What is OFDM?� Modulation technique
�Requires channel coding�Solves multipath problems
OFDM modulation
Source coding
I/Q-mod., up-
converter
I/Q RFChannel coding /
interleaving
Transmitter:
InfoSource
OFDM de-modulation
Source decoding
Down-converter,
I/Q-demod.I/Q RF
Decoding / deinter-leaving
Receiver:Radio-channel
InfoSink
e.g. Audio 0110 01101101
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Multipath Propagation
� Reflections from walls, etc.
� Time dispersive channel� Impulse response:
� Problem with high rate data transmission:� inter-symbol-interference
τ [ns]
p (τ) (PDP)
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� �����������
Inter-Symbol-InterferenceTransmitted signal:Received Signals:
Line-of-sight:
Reflected:
The symbols add up on the
channelDelays
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Concept of parallel transmission (1)Channel impulse response
1 Channel (serial)
2 Channels
8 Channels
Time
In practice: 50 … 8000Channels (sub-carriers)
Channels are transmitted at different frequencies (sub-carriers)
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The Frequency-Selective Radio Channel
� Interference of reflected (and LOS) radio waves
�Frequency-dependent fading
-10
-5
0
5
10
15
20
Frequency
Power response [dB]
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Concept of parallel transmission (2)Channel impulse response
1 Channel (serial)
Channeltransfer function
Channels are “narrowband”
2 ChannelsFrequency
Frequency
8 ChannelsFrequency
FrequencyTime
Signal is “broadband”
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Concept of an OFDM signal
Ch.1
Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10
Saving of bandwidth
Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10
Ch.1
Conventional multicarrier techniques
Orthogonal multicarrier techniques
50% bandwidth saving
frequency
frequency
20 ©Mohamed Khedr., 2008
A Solution for ISI channels
• Conversion of a high-data rate stream into several low-rate
streams.
• Parallel streams are modulated onto orthogonal carriers.
• Data symbols modulated on these carriers can be recovered without mutual interference.
• Overlap of the modulated carriers in the frequency domain - different from FDM.
21 ©Mohamed Khedr., 2008
OFDM
• OFDM is a multicarrier block transmission system.
• Block of ‘N’ symbols are grouped and sent parallely.
• No interference among the data symbolssent in a block.
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OFDM Mathematics1
2
0
( ) k
Nj f t
kk
s t X e π−
=
= � t ≡ [ 0,Τos]
Orthogonality Condition
*1 2
0
( ). ( ) 0T
g t g t dt =�In our case
2 2
0
. 0p q
Tj f t j f te e dtπ π− =�
For p ≠ q Where fk=k/T
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Transmitted Spectrum
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resemblesIDFT!
Spectrum of the modulated data symbols� Rectangular Window of
duration T0
� Has a sinc-spectrum with zeros at 1/ T0
� Other carriers are put in these zeros
� � sub-carriers are orthogonal
Frequency
Magnitude
�−
=
−∆−=1
0
)(2,, )()(
N
i
kTtfijkikBB exkTtwts π
N sub-carriers:
T0
25 ©Mohamed Khedr., 2008
OFDM terminology
• Orthogonal carriers referred to as subcarriers {fi,i=0,....N-1}.
• OFDM symbol period {Tos=N x Ts}.
• Subcarrier spacing ∆f = 1/Tos.
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OFDM and FFT
• Samples of the multicarrier signal can be obtained using the IFFT of the data symbols - a key issue.
• FFT can be used at the receiver to obtain the data symbols.
• No need for ‘N’ oscillators,filters etc.
• Popularity of OFDM is due to the use of IFFT/FFT which have efficient implementations.
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Interpretation of IFFT&FFT
� IFFT at the transmitter & FFT at the receiver
� Data symbols modulate the spectrum and the time domain symbols are obtained using the IFFT.
� Time domain symbols are then sent on the channel.
� FFT at the receiver to obtain the data.
28 ©Mohamed Khedr., 2008
Interference between OFDM Symbols
• Transmitted Signal
• Due to delay spread ISI occurs
Delay Spread
IOSI
OS1 OS2 OS3
• Solution could be guard interval between OFDM symbols
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Cyclic Prefix
• Zeros used in the guard time can alleviate interference between OFDM symbols (IOSI problem).
• Orthogonality of carriers is lost when multipath channels are involved.
• Cyclic prefix can restore the orthogonality.
30 ©Mohamed Khedr., 2008
Cyclic Prefix Illustration
TosTg
Cyclic Prefix
OS 1 OS 2
OS1,OS2 - OFDM Symbols
Tg - Guard Time Interval
Ts - Data Symbol Period
Tos - OFDM Symbol Period - N * Ts
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������������� ���� �������������
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Design of an OFDM System
Channel impulse
response
Guard intervallength
Channel Parametersare needed
x(4 … 10) FFTsymbollength
Nr. ofcarriers
Data rate;modulation
order
Other constraints:•Nr. of carriers should match FFT size
and data packet length•considering coding and modulation schemes
33 ©Mohamed Khedr., 2008
Advantages of OFDM� Solves the multipath-propagation problem
�Simple equalization at receiver� Computationally efficient
�For broadband systems more efficient than SC� Supports several multiple access schemes
�TDMA, FDMA, MC-CDMA, etc. � Supports various modulation schemes
�Adaptability to SNR of sub-carriers is possible
34 ©Mohamed Khedr., 2008
Problems of OFDM (Research Topics)
� Synchronization issues:
� Time synchronization� Find start of symbols
� Frequency synchr.
� Find sub-carrier positions
� Non-constant power envelope
� Linear amplifiers needed
� Channel estimation:
� To retrieve data
� Channel is time-variant
0 20 40 60 80 100 120 140 160 180 200-0.2
-0.1
0
0.1
0.2time domain signal (baseband)
sample nr.
imaginaryreal
δf frequency offset
frequency
amplitude
35 ©Mohamed Khedr., 2008
Serialto Parallel
X0
XN-1
x0
xN-1
IFFT
Parallel to
Serialand
add CP
Add CP
WindowingDACRF Section
InputSymbols
OFDM Transmitter
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ADCandRemoveCP
Serial toParallel FFT
Parallel to Serialand Decoder
X0
XN-1
x0
xN-1
OutputSymbols
OFDM Receiver
37 ©Mohamed Khedr., 2008
Synchronization
• Timing and frequency offset can influence performance.
• Frequency offset can influence orthogonality of subcarriers.
• Loss of orthogonality leads to Inter Carrier Interference.
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Peak to Average Ratio
• Multicarrier signals have high PAR as compared to singlecarrier systems.
• PAR increases with the number of subcarriers.
• Affects power amplifier design and usage.
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40 ©Mohamed Khedr., 2008